The present invention relates to a filter apparatus and a filtering method, more particularly, to a filter apparatus and filtering method with which non-Newtonian fluids such as magnetic coating solutions that have thixotropy and other time-dependent effects can be filtered before they are applied onto flexible bases.
Magnetic disks used as file memories for electronic computers and magnetic tape cassettes for use on VTRs are examples of magnetic recording media in common use that are manufactured by coating magnetic coating solutions on various types of bases including flexible tapes and rigid bases.
Magnetic coating solutions used in the production of such magnetic recording media are conventionally prepared by a process involving a step of mixing with a ball mill, a kneader, a sand grinder, etc. Because of the mixing step involved, the magnetic coating solutions contain not only balls and other worn particles that have occurred during mixing but also foreign matter that results from incomplete dispersion or reagglomeration of magnetic particles, resins, etc. In the prior art, these unwanted materials are eliminated by passing the magnetic coating solutions through a filter as appropriate before they are applied onto bases to form coated layers.
While various coating methods are known, high production rates have been achieved by performing continuous coating on a running flexible base with an applicator using reverse rollers or an extrusion-type applicator.
Two examples of a conventional filter apparatus are shown in FIGS. 6 and 7. As shown therein, a plurality of tubular filter cartridges 40 are juxtaposed in a filter housing 30, and a fluid passing through those filters is collected in the secondary space 31 of the housing before it is pumped through a predetermined piping system.
However, given the configuration of the secondary space 31 shown in FIGS. 6 and 7, the flowability of the coating solution may be low in a certain area, for instance, hatched region X, for the solution in entering the secondary space 31 compared with the flowability of the fluid in exiting the secondary space 31, depending on the position of an inlet 32 for the fluid coming from filter relative to the position of an outlet 33. If this problem occurs with a thixotropic coating solution that has a comparatively high solids content, the solution is prone to experience reagglomeration of particles into clumps in that region of low fluidity.
The higher the apparent viscosity of the coating solution, the greater the chance of it coagulating in the "dead space" of the secondary region (where fluidity is reduced). It may be safely assumed that the agglomerate is dislodged in portions and carried away by the flow of the coating solution. The agglomerate in the effluent acts substantially as foreign matter in the subsequent coating step (i.e., the coating solution is deposited on a base by passage through a coating head), thereby causing streaking and other defects on the coated surface of the base.
If a coating solution containing such an agglomerate is used to form a magnetic recording layer as in a magnetic tape, the resulting recording medium will have poor recording and reproducing characteristics, as typified by the occurrence of many dropouts during recording or reproduction.
It might be thought that this problem could be solved by designing a housing of such a configuration that there is no dead space in the secondary region 31; however, it is impossible as a practical matter to eliminate the dead space completely. The region of the dead space undergoes subtle changes depending upon various conditions such as the relative positions of the inlet through which the filtrate enters the secondary space and the outlet through which the filtrate emerges from the secondary space, the velocity of fluids and the viscosity of the coating solution. In particular, with fluids such as magnetic coating solutions that have a thixotropic effect (the viscosity changes in response to the shear force applied to the fluid), it has been very difficult to provide a filter apparatus that permits the dead space to be reduced under all conditions at all times merely by adopting a certain configuration. Under these circumstances, it has been common practice to determine the configuration of the housing of a filter apparatus individually on an empirical basis.